(i) Field of the Invention
The present invention relates to a polypropylene resin composition, and more specifically, it relates to a polypropylene resin composition from which there can be manufactured, under a good moldability by a calender molding machine, films or sheets which have less thickness unevenness, excellent appearance and heat resistance and such flexibility as in molded articles of a plastic polyvinyl chloride and which are useful as civil engineering construction materials and automobile interior covering materials.
(ii) Description of the Related Art
A calender molding method is a technique in which a resin composition is rolled between two rolls to continuously form films or sheets having a constant thickness, and it has some advantages. For example, the production power of the calender molding method is higher as compared with that of a usual extrusion molding method, and so the films or the sheets can be supplied at low costs in large quantities to markets. In addition, the thickness precision of the obtained films or sheets is high, and therefore winding wrinkles scarcely occur on the sheets.
Heretofore, a large number of leathers, sheets, films and the like have been manufactured from a plasticized polyvinyl chloride by the use of the above-mentioned calender molding method, but in recent years, it has been demanded to use another material in place of the plasticized polyvinyl chloride owing to a problem of the poison of a plasticizer and a problem of the generation of a harmful gas at the time of the combustion of wastes.
On the other hand, polyolefin resins such as polyethylene and polypropylene do not have the above-mentioned problems and are excellent in physical and chemical characteristics, and therefore they have been used in a wide variety of fields. However, these resins do not have such flexibility as in the plasticized polyvinyl chloride, and when they are molded by the calender molding method, thickness unevenness occurs in the obtained sheets or films. In addition, these resins have another drawback that if the selection of a lubricant to be added is mistaken, plate-out takes place on the surfaces of calender rolls and the discharge of the lubricant (a blooming phenomenon or a bleeding phenomenon) occurs on the surfaces of the molded articles.
Furthermore, a material for calender molding in which a polyolefin resin is used has been disclosed (Japanese Patent Application Laid-open No. 26077/1995), but the basic resin of this material is a usual ethylene-propylene copolymer rubber. Accordingly, it has a drawback that heat resistance is low.